Production of hot-pressed hardboard



United States l atent 2,757,113 PRODUCTION OF HOT-PRESSED HARDBOARD George B. Matter, In, Longview, Wash, assignor to Weyerhaeuser Timber Company, Tacoma, Wasln, a corporation of Washington No Drawing. Application November 8; 1954', Serial No. 467,625

18 Claims. (Cl. 154-1b1) The present invention relates generally to the manufacture of consolidated fiber bodies by hot-pressing fiber felts, and more particularly to the use therein of air-laid felts characterized by lack of an appreciable quantity of water expressible during hot-pressing.

Heretofore, such products have been made by form-. ing water-laid felts, pressing these to express excess water thereby leaving wet felts, then pressing and consolidating the wet felts with or without added binder in a press employing heat and pressure. In this so-called wet process. a serious deficiency is the loss of water-soluble material of the original fiber stock to the white water which is drained away in forming the wet felt. When the Wet felt is subjected to the initial stages of hot-pressing, the moisture content is such that there is an appreciable. amount of water squeezed out of the felt, and this Water carries away an additional. quantity of water-solubles. Nevertheless, one advantage accruing from such losses of water-solubles is minimizing the danger of staining the product in hot-pressing, as will appear hereinafter and as such is purposely included as a process step by many manufacturers. The loss of solubles, however, is uneconomic and desirably to be minimized and preferably avoided, because of lowered product yield and particularly to avoid loss of the bonding properties of the. solubles. The processing costs are high when using water in felt.- ing. Because of the high content of residual Water in the felt to be hot-pressed, and because substantially all the water present in the mat must be removed by in partexpressing it or wholly or in part by volatilizingit, high costs are encountered in manufacture of a hot-pressed; product.

There are ways to overcome one or more of these disadvantages. Improved yield of product is achieved by use of fiber in which a large proportion or all of the Watersolubles have been retained, and in particular by use of fiber which contains substantially all the substance of. the natural lignocellulose from which the fiber is derived. The common. case is. wood, from which such fiber may be prepared innumerous ways. Loss of solubles by being expressed with the water in felt formation may be obviated by forming the felt in the absence of suspending water, as by air-laying it. Numerous methods and apparatus are available for such practice. Any selected procedure may dictate a range of and a favorable value for themoisture content of the fiber to be felted, or of the resulting felt. The use of whole natural lignocellulose and the air-laying technique are sometimes combined.

Preferred practice in production of consolidated fiber products calls for directly hot-pressing an air-laid felt of fibers containing moisture but not wet with water. However, where moisture contents are correctly chosen, additional. water may be added advantageously to a felt to be hot-pressed, as for example, to one or bothsurfaces of the felt with disregard of uniformity of distribution.

In so consolidating air-laid felts of the character described, namely, those containing fibers having Watersolubles in natural proportion, or in augmented propor- 2,757,113 Patented July 31, 1956 tion where limited steaming of the wood substance has been practiced, the pressing conditions used heretofore have been such that staining of the face or faces to an undesirable extent has resulted. Adding water to the felt as. suggested above increases the tendency to stain. One means used to minimize staining where steam. treatment of the wood substance has been employed to facilitale preparation of the, fiber from wood, has been so to control and limit the steaming that creation of watersolubles in the felt is minimized.

The present invention aims to prevent such staining by use of controlled stepwise operations in hot-pressing, thereby enabling a practical disregard of the amount of water-solubles inthe. felt, and further in disregard of the above-mentioned added water which in the presence of such water-solubles induces staining, yet which is advantageous for creating a superior quality of surface.

It is a general, object of the invention to employ for consolidation. a felt of fibers which contain. natural or augmented quantities of bonding water-solubles, and to apply and then to relieve mechanical pressure during the consolidation operation at a predetermined stage, and thus to eliminate one of the causative factors of staining, whereby it, does not. occur.

It is a particular object of the invention to use air-laid felts of whole wood fiber containing water-solubles of wood origin which are. natural to the wood, and permissibly to augment. said material amount by action of steam, and so to conduct. the. hot-pressing operation that staining is avoided.

' It is alsovv an object of the. invention to use air-laid felts of wood fibers containing. substantially all the organic material of the wood from which the fibers are derived, which. felts: are. so. limited in total. moisture content that inthe compression for consolidation, substantially no water is expressed, whereby all. the water-soluble content deriving from said Wood is preserved for its bonding funcion in the consolidation.

It is still another object of the invention to use such air-laid felts of which the interior ismoist but not wet, and in which a surface selected to become a quality face is wetted by controlled addition of water just before the hot-pressing.

Various other and ancillary objects and advantages of the invention will appear from the following description and explanation.

The invention hasbeen more completely practiced with fibers of Douglas fir and. of white fir (Abies' concolor) prepared: in the Asplund defibrator by the process of U. S. Patent No. 2,008,892, which involves the presence of steam and incidentally the generation of water-solubles in amount generally increasing as the time or tempera tare, or both, of the exposure to steam increases. It has alsobeen practiced. on fiber prepared mechanically from raw wood with no treatment to increase its content of water-solubles above the natural content. In both cases, the water-solubles have an interfiber bonding effect which is pronounced. The degree of interfiber bonding may be further increased if desirable by' use of added binders, such as thermosetting resins. Such added binders are expensive. However, usage of; such added binder may be ap reciably minimized. by special endeavor to increase the content of water-solubles deriving from the wood itself and by preserving the same with the fibers for use in the consolidation. Heretofore water-solubles in the fiber mat have been minimized to avoid surface staining which they cause as a result of consolidation by hot-pressing.

The steaming time in the normal Asplund defibration is relatively short. In this process, as it is commercially practiced to produce fiber economicaiiy from Douglas fir, for example, the steam pressure is in the range from 50 to 200 p. s. i. g. (pounds per square inch gauge pressure) and the wood substance is exposed to it for a time usually of about one minute, but permissibly from 6 to 0.5 minutes, respectively, but not over 2 minutes at 200 p. s. i. g. In the Asplund process, wood chips are fed into a high pressure steam chamber wherein the chips are quickly softened, and wherein the softened chips are mechanically rubbed to fiber by relatively rotating disks. From the periphery of these disks the fiber is discharged to a gaseous atmosphere of lower steam pressure or to the atmosphere.

The time and temperature of the steaming in the said defibrator advantageously may be each or both increased in order to increase the water-solubles deriving from the wood itself, being careful, however, to shorten the maximum time as temperature increases so as to avoid at 200 p. s. i. g. (388 F.) 'an exposure of over 2 minutes. These are about the critical conditions for gasifying and losing wood substance by generation of furfural and other non-bonding chemical substances, which conditions also lead to dark colored fiber. Neither the sequence nor concurrency of steaming and defibering is critical. One may follow the other, but the Asplund process is preferred.

Such fibers are then air-laid to a felt by any suitable process, with moisture control at any appropriate stage to yield mats having a uniform moisture content in the range from 8% to 35% by weight. The lower limit is imposed arbitrarily to facilitate felting and to eliminate deleterious effects such as those caused by static electricity. The upper limit is imposed to permit increase of total moisture content by wetting one or both faces for hot-pressing to produce glaze. When the total moisture content exceeds 40%, it is very difficult to consolidate the felt by heat and pressure and retain sufficient water-solubles to effect glaze, because the amount of water lost by expression carries off too large an amount of the water-solubles. However, when the total moisture content is not over an amount in the range from about 24% to about 28%, and sometimes 30%, no water is lost from an assembly in a practical cycle of hot-pressing to form a board of about 64 lbs. per cu. ft. density, when the felt is located between a smooth caul and a wire screen. So many factors are involved in determining that critical content of moisture at which water is expressed from the mat in hot-pressing, that it is not possible to specify a universal maximum.

By allowing a small loss of water-solubles as a result of some expression of water from the mat, a maximum of 40% moisture content is generally applicable. In fact, experience has shown that by wetting the fibers at a face to be glazed, the wetting being essential, loss of some water by expressing it from the assembly is difficult to avoid when a satisfactory glaze is produced. in practice for assemblies having a screen at one face and a smooth caul at the other, the total moisture content of the felt varies from 30% to 40%, in making boards having a density of 64 lbs. per cu. ft. The amount of water-solubles lost is generally of the order of 10% or less of the total content of water-solubles. By increasing the steaming action on wood by which water-solubles are created, the loss, even if greater, is easily compensated for.

Whether or not a felt contains expressible water in hot-pressing depends among other factors upon the total moisture content, upon its distribution, and also, importantly, upon the density and temperature of the felt at the critical point where expressibility comes into existence. The density and the temperature vary with the pressing cycle as to time, mechanical pressure, temperature, and rate of supplying heat. In addition to these factors, is the character of the assembly, such for example, as the presence or absence of a screen-facing against the felt being pressed. The particle-size-distriv bution of the felt contents is also important in controlling the volume ratio of capillary spaces to total volume.

The previous history of the fibers, and the presence or absence of additives, such as water-resisting agents, affect the capacity of the fibers to hold water, and also the rates of absorbing it and of yielding it.

When a felt has a uniform distribution of its moisture content, the pressing process first heats the faces before heating the interior, with the result that volatilization of water from the faces increases the capacity of the face layers to take up water which might actually be expressed from the cooler interior. Such a felt as a whole may therefore contain an inexpressible content of water for a particular pressing cycle, even though the central layer thereof, if isolated, might lose water by expressing it therefrom. In the case where only one face of the felt lies against a screen and the other face is confined against a smooth caul, expressible water is lost earlier in a given pressing cycle (i. e., at a lower effective mechanical pressure) than in the case of an assembly of the same felt wherein both faces are confined "against smooth cauls.

In those embodiments of the present invention wherein one or both of the face layers are sprayed so as actually to wet the fibers, a reduction of water content of the mat is likely first to take place at the wet face layer against a smooth caul. Generation of steam pressure at the more highly heated face layers tends to drive at least some of any locally expressed water toward the interior. Such water will move into the capillary spaces and also will be absorbed by the fibers which have capacity to absorb more water. Accordingly, the pressing cycle should be carried out sufiiciently slowly to avoid reducing the capillary volume so fast that it cannot hold such water as may be expressed from wetterl face layers against smooth cauls.

Each felt has a maximum capacity to hold water for each type of assembly and for each combination of mat density and temperature, but in a pressing cycle of the type herein employed, the mat temperature is not uniform as the density is changing. In every pressing cycle lacking the expression of water, all the water removed from the felt is lost as vapor. The pressing cycle must be carried out with regardto the variable factors in the mat assembly such as the screen face, the total moisture content and its distribution. These adjustments of con ditions in a commercially practicable pressing cycle for limiting the loss of water for the purposes desired may be readily effected when the total moisture content does not exceed 40% by weight; and avoiding loss of water may be effected in such a cycle using assemblies with one screen and a wet face against a smooth caul, and platen temperature of 360 B, when the total moisture content lies in the approximate range of 24% to 28%, and sometimes 30%, but such limitation does not mean that this range is an absolute criterion of inexpressible water for every possible pressing cycle.

For the highest quality of product it is important that the mat have substantially the same amount of wood substance at every unit area. This may be achieved by forming a mat substantially uniform in thickness, in density, and in moisture content. Variation in these factors within a mat, when conventional continuous hotpressing technique is used easily leads to staining by forming in the press local areas of relatively higher resistance to flow of vapor through the mat. However, by practice of the present invention substantial deviations from uniformity may be permitted without resulting in staining.

A typical mat ready for pressing has, for example, a uniform oven-dry fiber density of 3 pounds per cubic foot, is 2.6 inches thick, and has a uniform moisture content of 25%. Its fibers consist of whole wood substan tially all in the form of ultimate fibers and opened up aggregates of ultimate fibers, as may be produced by the said defibrator process, with or without some additional size reduction of any content of larger sizes which may be. $0 produced along with the ultimate fibers. Such a felt may be placed with one or both faces against smooth caul plates or heated platens for consolidation to produce one or two smooth faces in the resulting product.

Experience has shown that when these faces which are so intended to be smooth are made actually wet, as by spraying with water just before pressing, a desirable glaze is produced when using a smooth platen or caul plate. Otherwise, in mats not so wetted the face is duller, more matte in. appearance and of a fibrous texture. In a.pressing cycle of conventional pressing, such. a glazed face or such a matte face is stained with areas of dark color and with much darker edges of the stained area. The stain is more extensive at wetted faces. The stained areas are located remotely from the edges. The said conventional practice involves heated platens having a temperature in the range from 300 to 450 F., preferably 400 F. In the practice of the presentinvention, the transmission of heat to the mat is controlled. This may be done by using insulated platen faces, or where the platen face is not insulated from its source of heat, imposing insulation between it and the felt being consolidated. The reasons for this will appear in connection with the explanation which follows.

A certain press having uninsulated smooth faced platens at 400 F. is operated on an horizontal assembly consisting upwardly of: insulating fiber sheet, metal caul plate, wire mesh screen, the typical mat above described with its top face wetted to provide a total moisture content for the mat to not over 40%v byweight, smooth caul' plate, and insulating fiber sheet, Every press. has mechanical limitations which limit its flexibility. The press referred to will produce staining on the described assembly when operated to produce a of 64 pounds per cubic ft. by the following time cycle:

Time to close onto an assembly, up

to 15 seconds. Time to compress assembly to reach 48 p. s. i to seconds. Time to compress at 350 p. s. i to seconds. Time to reach 750 p. s. i 20 seconds. Time to hold for cure at 750 p. s. i 2 minutes. Time to open press and remove assembly Not significant;

The removed product in general will show a perimetric area at the facial edges of the panel (4 feet square) of excellent appearance having a glazed surface which is free from stain. Around the central area of the glazed face may be one or more small or large islands of stain, dark in appearance against the lighter edges, and much darker on the shores of the islands. The stain commonly extends from face to face, but not uniformly. Analysis shows the stained area to be higher in water-solubles than unstained area, and shows the concentration of the water-solubles to be heaviest in the face layer of the stained area and decreasing inwardly. Studies of the conditions, and experiments to overcome such staining in arriving at the present invention, have led to a probable theory as to the mechanism. The present invention as a remedial practice fits the theory.

The following explanation is ofiered without any intention to become committed to it or that it be a limitation to be imposed on the present invention:

In the presence of moisture in the felt, the applied mechanical pressure and the retarded transmission of heat work together initially to change conditions in the mat. The pressure compacts the felt to a body of low porosity and of high resistance to flow of gas (air and water vapor) through it, as the heat more slowly raises the temperature of the water content. The moisture or free water (of spraying) is a part vaporized to steam within' the previously compressed body, some vapor escaping through the edges, and much of it being partially con fined within the body at increasing water-vapor pressure in. directions inwardly from the edges. As the. pressing board having a density adiabatic expansion marks the critical time. Prior to this.

continues bothvapor and aqueous liquid exist within the compressed body at increasingly higher temperatures as the distance inwardly increases, the liquid being increasingly solvent of material in the body as the temperature increases. The mat temperatures continue to build up after maximum mechanical pressure and minimum thickness have been achieved. As the temperatures build up, the vapor pressure at the areas near the edges is lower than centrally because of loss of water vapor edgewise and because of inwardly increasing resistance to loss of vapor from the interior. Liquid in the edge areas may vaporize as fast as it acquires heat. However, centrally this is not so and the partially confined liquid becomes hotter at higher vapor pressure as the distance increases inwardly from the edges. When the boiling point is reached a steam pocket forms by boiling of the liquid, a condition indicated'by a rapid but slight fall in temperature, as may be observed by an implanted thermocouple. The steam pocket tends to force surrounding liquid in all directions away from the pocket, and of course it moves further in the directions of least resistance. Why it goes to the smooth caul plate is next explained.

While the press is held closed to effect the cure, steam gradually escapes, evidenced by the fact that moisture or water content is drastically reduced, and generally to less than 1% by weight in the case of high-density boards of to lbs. per cu. ft. There are three avenues of escape, namely, the exposed edges of the body, the body face on the screen, and thebody face against the smooth caul, plate. The evidence supports the statement that there is some facewise loss of vapor toward the smooth caul plate and a resulting escaping interfacial layer of water vapor between the caul plate and the compressed body. When a steam pocket forms, some liquid moves.

toward the smooth caul plate and into the interfacial vapor layer with outward spreading within said interface toward the edges, while most of it moves within the mat toward the edges and downwardly toward the screen. Be cause of the greater vaporization at the screen face, the interstitial bodies of fluid in the mat are more concentrated in solids toward that face, and hence less fluid. The water content of the moved fluid vaporizes more freely at the interface, being nearer the source of heat, and thus concentrates the solids therein as an area of stain. The edges of the liquid at the interface also vaporize water into the interface, drawing more of the liquid to replace it, thus leaving more residue to form the darker edges of the stain as described In practice of the present invention, the assembly may be as above described, but as soon as the press has been closed to establish a density in the vicinity of final density, and preferably substantialiy final density and thickness, and before the mat has become uniformly heated throughout, the pressure is relaxed to a value such that the internal vapor pressure, highest in the generally stainable area, is relieved facewise from the mat toward the caul plate, where it escapes as the interfacial layer described. Sufiicient water is thus removed from the body so that su stantially no interstitial fluid body remains. The. required time and extent of such relief or reduction in mechanical pressure varies with numerous variable factors, such as board density, particle size and amount and kind of additives, and are readily determined by instruments or by cut-and-try methods for any set of fixed conditions. The release of vapor need notbe effected in a single continuous step, and may consist of a series of intermittent steps. Avoidance of a boiling condition internally prevents the staining and provides a uniform facial appearance. By this procedure, deposited Water-solublcs remain in situ uniformly over the surface.

Given any set of fixed conditions the time required to generate the said coercive steam pocket is readily to be found by use of an imbedded thermocouple. The described slight drop in temperature which results from its '7 critical time, but after compressing the mat substantially to final density, the pressure is relaxed a predetermined amount but not wholly released for a time or times to bleed out excess vapor, and to obviate flow of solution in the body.

Numerous factors predetermine the rate at which heat from the platens source should be conducted to the mat. This may be controlled by insulation between the mat and the source of heat in the platen. Where a caul plate is used to control the texture of a mat face, an insulator, such as a wire screen, must lie between the caul plate and the source of heat. When there is a screen between the mat and the caul plate, it functions also to facilitate removal of water vapor from the mat, and thus shortens the pressing cycle, but this assistance to exit of water vapor through the screen is not sufficient under conditions of continuous pressing to prevent staining on the opposite side against a caul plate, even in consolidated products only /8 inch thick.

In production practice the building of insulated assemblies to be pressed is avoided by incorporating insulation into the press. Thus, on a platen face directly heated from its interior, there is placed, for example, a wire mesh screen of chosen porosity and resistance to deformation in use, and this is capped by a suitably thick metal sheet, preferably smooth, which becomes the platen face which contacts the mat. Upper and lower platens may be insulated to the same degree or different degrees, according to the assembly to be used. With such insulated platens, the assembly may be of at least two kinds. A board having but one smooth face (S-l-S meaning smooth on one side) is made from an assembly consisting vertically of supporting caul plate, wire mesh screen, and moist mat. A board having two smooth faces (S2S meaning smooth on two sides) is made from an assembly consisting vertically of supporting smooth caul plate and moist mat, presupposing in each case the upper insulated platen face to be smooth.

The following examples are suitable cycles for assemblies 52 inches square containing insulating sheets for the specific press above referred to, to change the result from a staining to a non-staining operation. The size of the assembly is given as it predetermines timing in the cycle, by predetermining the distance from the most remote point to an edge. The fibers are whole wood Douglas fir defibered in the Asplund defibrator at 160 p. s. i. g. for 60 seconds with added binder of about 1 part of phenol-formaldehyde resin solids per 100 parts of oven-dry fiber.

As is well known in the art, heating in the press performs two functions among others. The primary function is to consolidate the mat. The second function is to reduce the moisture content. The final moisture content must be such that on opening the press the vapor pressure within the consolidated product will not rupture or blister the structure. In high density boards, the final moisture content must be lower than in similarly bonded boards of lower density. However, the kind and degree of bonding at each density of product predetermines the critical moisture content at which rupture or blistering may occur.

Although the following examples are based upon use of insulation in the press, the invention is not so limited. The idea is to retard the initial transmission of heat to the felt. Use of insulation between already heated platens and a felt between them, with or without relatively thin cauls, is merely a practical measure. Another expedient, eliminating insulation, is to place the mat between much thicker and cold metal cauls and then subject such assembly to compression between already heated platens. The cold cauls must first be heated before the mat is effectively heated. The caul material, thickness and initial temperature are factors for regulation of heat transfer as desired. Another factor of like function is the addition, within limits as to quantity, of water at the face or faces of the mat. Another factor, related to both insulation and cauls having predetermined heat capacity to be satisfied, is the speed with which press platens act on the 'mat in reaching substantially the final thickness of intended consolidation. A less practical method is to compress directly between initially cold platens and then control the rates of heating of the platens and delivery of heat therefrom to the mat.

It is to be understood that it is not necessary for each face of the mat to be identically heated. For example, in a continuously used batch-type multi-opening press, the upper platen may be insulated, and the mat may enter upon a cold, thick metal carrying-caul.

Example 1 (S1-S) Y The press has two rams, a light one to move it and a heavy one to exert the greater pressure. in operation,

the pressing is:

Seconds To eliminate open space between platens and assembly 15 To reach 48 p. s. i. with light ram 5 to 10 To compress at 48 p. s. i 15 to 25 To increase pressure to 75 0 p. s. i. with the heavy ram 20 To hold at 750 p. s. i 45 To relax pressure from 750 to 48 p. s. i 30 To hold at 48 p. s. i 30 To increase to 60 p. s. i 30 To increase to p. s. i 30 To increase to 750 p. s. i 15 To hold at 750 p. s. i 45 To open and relieve all pressure 30 The product is a panel Vs inch thick at 64 pounds per cu. ft.

Example 2 (S2S) A panel similar to that of Example 1, but smooth and stain-free on two sides is made as follows:

Assembly: Transport caul, fiber insulating sheet, bottom smooth caul plate-mat 2.6 inches thick at 3 lbs. oven-dry fiber per cu. ft. with uniform mat moisture of 24% by weight and an additional spray on each face of ounces of water per sq. ft. giving a total of 36% moisture-smooth caul plate, and fiber insulating sheet. Platen temperature 400 F.

Seconds To eliminate open space betwen platens 15 To reach 48 p. s. i 5 to 10 To compress at 48 p. s. i 15 to 25 To increase pressure to 750 p. s. i 15 To hold at 750 p. s. i 60 to 150 To reduce pressure to 30 p. s. i To hold pressure at 30 p. s. i 360 To increase pressure to 750 p. s. i 15 To hold pressure at 750 p. s. i 45 To open and relieve all pressure 30 The product is an S2S panel inch thick at 64 lbs. per cu. ft.

Example 3 (SJ-S) A panel thinner and more dense than that of Example 1 is made by using the same assembly and mat as in Example 1, the mat being formed with 25% by weight of moisture, and using a spray of water in the amount of ounces per 100 sq. ft. giving a total moisture content of 36.4%. A suitable pressing schedule with platens at 400 F. is:

- Seconds To eliminate open space between platens 15 To reach 48 p. s. i 5 to 10 To. compress at 48' p. s. i 10 to 15 To reach 750 p. s. i 15 to 20 To hold at 750 p. s. i 30 To reduce to 48 p. s. i 15 To increase to 60 p. s. i 10 To hold at 60 p. s. i 10 To reach and hold 80 p. s. i 10 To reach 750 p. s. i 10to: 15 To hold at 750 p. s. i 105 to 110 To open and relieve all pressure 30 The product is an S-l-S panel $4 inch thick at 68 lbs. per cu. ft.

Example 4 (S-I-S} A panel thicker and less dense than that of Example 1 is made by using the same type of assemblyas in Example 1, but a mat 4 inches thick at 2.4 lbs. oven-dry fiber per cu. ft., and an initial moisture content of 25% by weight with a spray of Water on one face at 180 ounces per 100 sq. ft. giving atotal moisture content of 32%. A suitable pressing schedule with platens at 400 F. is:

Seconds To eliminate open space between. platens 15 To reach 48 p. s. i 15 to 20 To hold at 48 p. s. i 25,to.3.0 To reach 560, p. s. i 10 to 15 T hol at 56.0 p. s. i v 5 Toreduce to 30p. s. i 30 To hold at 30 pa. i r 540 To reach 100 p. s. i' ,r v 15 To open and relieve. all pressure 15 The product is an 8-16 panel /4 inch thick at 55 lbs. per cu. ft. 7

Example 5 (S .ZS)- A non-glazed S-2'S panel similar to that of Example 1 is made using the assembly of Example 2 and a mat as in Example 1, but of lower moisture. content, namely, 12% by Weight. A suitable schedule with. platens. at

Theproductisan S -21-S panel of fibrous texture on both faces inch thick at 64 pounds per cu. ft;

The following; examples of boards with. unstained faces relate to use of a different; press operating with different controls and with diiferent assemblies. The mat is carried into the press on a caul which retains heat from a prior use in the press in: a. process duplicating the respective examples given, below, and having a temperature ranging upwardly from, room temperature to one under 150 F. The upper caul issecuredto the upper platen and therefore is always; heated as: it approaches and meets the, mat. Both: platenshave an-internal tern: perature of 360 F. In the case of 8-1-8 boards. awire screen is interposed between the carrying caul. and the mat. The screen facilitates lOSs. of water which attends the conditions necessary to produce ans-159 board with glaze on th y upper face as, a: resulttof' wettingthe upper layer ofthe: mat.

Example 6 (S-I-S) A fiber felt is formed having an average of 14% moisture content and varying generally in the range from 12% to 16%. The mat unit to be pressed Weighs 48.06 lbs. to form a board 4 ft. x 16 ft. x /s inch. The mat is sprayed on both faces with a total of 18 lbs. of water, distributed in the amounts of 3 oz. and 1.5 oz. per sq. ft., respectively on the top and bottom faces of the mat, sufficiently prior to entry into the press, so that the faces remain wet. This provides a total moisture content of 37.4%.

The mat is pressed with the following cycle:

Seconds At 500 p. s. i 30 Release to 50 p. s. i 30 At 50 p. s. i to At 500 p. s. i 60 In the pressing a slight amount of water is lost largely through the screen facing including some of the free water adjacent the same. The smooth upper caul and the cycle effect unstained glaze on the top face of the board, which has a density of 62 lbs. per cu. ft.

Example 7 (S2-S) Seconds At- 250 p. s. i 30 Release to 30 p. s. i 30 At 30- p. s. i 300 At 750 p. s. i 60 The resulting board is 4;. inch thick with a density of 62 lbs. per cu. ft., and it presents a high quality hard glaze on the top face.

The present application is a continuation-in-part of my prior application Serial No. 391,381, filed November 10, 1953, and now abandoned.

From the foregoing examples, it is apparent how the variable factors influence the timing and the operation of the pressing period. When conditions prevail other than those specifically illustrated, the pressing may be arranged accordingly, Within the scope of the invention as expressed in the appended claims.

I claim:

l. The method which comprisesforming a felt'of wood fibers of substantially uniform content of fiber material per unit area, said' fibers consisting of substantially all the substance of the wood from which the fibers are derived. and being substantially all in the form of ultimate fibers andopened-up aggregates of ultimate fibers, the felt having by Weight not more than about 40% total moisture content a portion of which is distributed substantially uniformly throughout the interior. fibers of the felt at a moisture content'in the range from 8% to 35% by weight, the mass of said fibers having in substantially uniform distribution at least the natural watersolublecontent deriving from said Wood; exposing the edges of said felt to the atmosphere" and during said exposureapplying mechanical pressure tosaid felt between imperforate pressing surfacesand thereby effecting a thick ness which is substantially the thickness at which the felt is to be finally consolidated, simultaneously transmitting heat through said surfaces toward the faces of said felt during said compression fromheat sources having temperatures in therange from 300 to 450 F., said thickness being attained" prior to the time the felt becomes uniformly heated throughout; holding said mechanical pressure for a short time while generated water vapor escapes from the felt toward and then facewise along said surfaces and also at the exposed edges of the felt, and the water-vapor pressure within the felt increases inwardly from the edges in the presence of a correspondingly increasing proportion of interstitial fluid consisting substantially of water and dissolved watersolubles, during which said short time the temperature of said fluid increases in the directions inwardly from the edges of the felt; lowering the applied mechanical pressure prior to the attainment of the boiling point of the hottest portion of said fluid, whereby to permit increased rates of vaporization of water from the faces of the felt and edgewise of the felt with deposition of said watersolubles in situ as a residue, thereby avoiding movement and migration of said fluid as would occur when partially confined bodies thereof reach the boiling point and boil to form steam in pressure pockets causing such migration; and when volatilization of water is substantially completed with a resulting deposition of solute in situ for use as binder solids, increasing the mechanical pressure to attain and maintain final thickness to bond the fibers in part at least by said residue of water-solubles and to reduce the moisture content to a value permitting full release of said pressure without rupture of the resulting consolidated body.

2. The process of claim 1 in which the fibers of the felt contain water-soluble content deriving from the wood by the action of steam.

3. The process of claim 1 in which the fibers are mechanically produced from wood in an environment of steam whereby the water-soluble content of the wood substance is increased by the action of the steam.

4. The method which comprises forming a felt of wood fibers of substantially uniform content of fiber material and of thermosetting resin binder per unit area, said fibers consisting of substantially all the substance of the wood from which the fibers are derived and being substantially all in the form of ultimate fibers and openedup aggregates of ultimate fibers, the felt having by weight not more than about 40% total moisture content a portion of which is distributed substantially uniformly throughout the interior fibers of the felt at a moisture content in the range from 8% to 35% by weight, the mass of said fibers having in substantially uniform distribution at least the natural water-soluble content deriving from said wood; exposing the edges of said felt to the atmosphere and during said exposure applying mechanical pressure to said felt between imperforate pressing surfaces and thereby effecting a thickness which is substantially the thickness at which the felt is to be finally consolidated, simultaneously transmitting heat through said surfaces toward the faces of said felt during said compression from heat sources having temperatures in the range from 300 to 450 F., said thickness being attained prior to the time the felt becomes uniformly heated throughout; holding said mechanical pressure for a short time while generated water vapor escapes from the felt toward and then facewise along said surfaces and also at the exposed edges of the felt, and the water-vapor pressure within the felt increases inwardly from the edges in the presence of a correspondingly increasing proportion of interstitial fluid consisting substantially of water and dissolved water-solubles, during which said short time the temperature of said fluid increases in the directions inwardly from the edges of the felt; lowering the applied mechanical pressure prior to the attainment of the boiling point of the hottest portion of said fluid, whereby to permit increased rates of vaporization of water from the faces of the felt and edgewise of the felt with deposition of said water-solubles in situ as a residue, thereby avoiding movement and migration of said fluid as would occur when partially confined bodies thereof reach the boiling point and boil to form steam in pressure pockets causing such migration; and when volatilization of water is substantially completed with a resulting deposition of solute in situ for use as binder solids, increasing the mechanical pressure to attain and maintain final thickness to bond the fibers inpart by said residue of water-solubles and in part by said thermosetting resin to reduce the moisture content to a value permitting full release of said pressure without rupture of the resulting consolidated body;

5. The process of claim 4 in which the fibers of the felt contain water-soluble content deriving from the wood by the action of steam.

6. The process of claim 4 in which the fibers are mechanically produced from wood in an environment of steam whereby the water-soluble content of the wood substance is increased by the action of the steam.

7. The method which comprises forming a felt of wood fibers of substantially uniform content of fiber material per unit area, said fibers consisting of substantially all the substance of the wood from which the fibers are derived and being substantially all in the form of ultimate fibers and opened-up aggregates of ultimate fibers, the felt having by weight not more than about 40% total moisture content a portion of which is distributed substantially uniformly throughout the interior fibers of the felt at a moisture content in the range from 8% to 35% by weight, the mass of said fibers having in substantially uniform distribution at least the natural water-soluble content deriving from said wood; exposing the edges of said felt to the atmosphere and during said exposure applying mechanical pressure to said felt between imperforate pressing surfaces of which one is in direct contact with the felt and between the other of which and the felt there is a wire-mesh screen, and thereby effecting a thickness which is substantially the thickness at which the felt is to be finally consolidated, simultaneously transmitting heat through said surfaces toward the faces of said felt during said compression from heat sources having temperatures in the range from 300 to 450 F., said thickness being attained prior to the time the felt becomes uniformly heated throughout; holding said mechanical pressure for a short time while generated water vapor escapes from the felt toward and then facewise along said surfaces and also at the exposed edges of the felt, and the water-vapor pressure within the felt increases inwardly from the edges in which the felt is to be finally consolidated, simultaneously transmitting heat through said surfaces toward the faces of said felt during said compression from heat sources having temperatures in the range from 300 to 450 B, said thickness being attained prior to the time the felt becomes uniformly heated throughout; holding said mechanical pressure for a short time while generated water vapor escapes from the felt toward and then facewise along said surfaces and also at the exposed edges of the felt, and the water-vapor pressure within the felt increases inwardly from the edges in the presence of a correspondingly increasing proportion of interstitial fluid consisting substantially of water and dissolved water-solubles, during which said short time the temperature of said fluid increases in the directions inwardly from the edges of the felt; lowering the applied mechanical pressure prior to the attainment of the boiling point of the hottest portion of said fluid, whereby to permit increased rates of vaporization of water from the faces of the felt and edgewise of the felt with deposition of said water-solubles in situ as a residue, thereby avoiding movement and migration of said fluid as would occur when partially confined bodies thereof reach the boiling point and boil to form steam in pressure pockets causing such migration; and when volatilization of water is substantially completed with a resulting deposition of solute in situ for use as binder solids, increasing the mechanical pressure to attain and maintain final thickness to bond the fibers in part at least by said residue of water-solubles and to reduce the presence of a correspondingly increasing proportion of interstitial fluid water and dissolved watersolubles, during which said short time the temperature of said fluid increases in the directions inwardly from the edges of the felt; lowering the applied mechanical pressure prior to the attainment of the boiling point of the hottest portion of said fluid, whereby to permit increased rates of vaporization of water from the faces of the felt and edgewise of the felt with deposition of said water-solubles in situ as a residue, thereby avoiding movement and migration of said fluid as would occur when partially confined bodies thereof reach the boiling point and boil to form steam in pressure pockets causing such migration; and when volatilization of water is substantially completed with a resulting deposition of solute in situ for use as binder solids, increasing the mechanical pressure to attain and maintain final thickness to bond the fibers in part at least by said residue of water-solubles and to reduce the moisture content to a value permitting full release of said pressure without rupture of the resulting consolidated body.

8. The process of claim 7 in which the fibers of the felt contain water-soluble content deriving from the wood by the action of steam.

9. The process of claim 7 in which the fibers are mechanically produced from wood in an environment of steam whereby the water-soluble content of the wood substance is increased by the action of the steam.

10. The process of claim 7 in which thermosetting resin binder is distributed uniformly among the fibers of the felt.

11. The process of claim 10 in which the fibers of the felt contain water-soluble content deriving from the wood by the action of steam.

12. The process of claim 10 in which the fibers are mechanically produced from Wood in an environment of steam whereby the water-soluble content of the wood substance is increased by the action of the steam.

13. The method which comprises forming a felt of wood fibers of substantially uniform content of fiber material per unit area, said fibers consisting of substantially all the substance of the wood from which the fibers are derived and being substantially all in the form of ulticonsisting substantially of mate fibers and opened-up aggregates of ultimate fibers, the felt having by weight not more than about 40% total moisture content a portion of which is distributed substantially uniformly throughout the interior fibers of the felt at a moisture content in the range from 8% to 35% by weight, the mass of said fibers having in substantially uniform distribution at least the natural water-soluble content deriving from said wood; exposing the edges of said felt to the atmosphere and during said exposure applying mechanical pressure to said felt between imperforate pressing surfaces each in direct contact with a face of the felt, and thereby effecting a thickness which is substantially the thickness at moisture content to a value permitting full release of said pressure without rupture of the resulting consolidated body.

14. The process of claim 13 in which the fibers of the felt contain water-soluble content deriving from the wood by the action of steam.

15. The process of claim 13 in which the fibers are mechanically produced from Wood in an environment of steam whereby the water-soluble content of the wood substance is increased by the action of steam.

16. The process of claim 13 in which thermosetting resin binder is distributed uniformly among the fibers of the felt.

17. The process of claim 16 in which the fibers of the felt contain water-soluble content deriving from the wood by the action of steam.

18. The process of claim 16 in which the fibers are mechanically produced from wood in an environment of steam whereby the water-soluble content of the wood substance is increased by the action of the steam.

References Cited in the file of this patent 

1. THE METHOD WHICH COMPRISES FORMING A FELT OF WOOD FIBERS OF SUBSTANTIALLY UNIFORM CONTENT OF FIBER MATERIAL PER UNIT AREA, SAID FIBERS CONSISTING OF SUBSTANTIALLY ALL THE SUBSTANCE OF THE WOOD FROM WHICH THE FIBERS ARE DERIVED AND BEING SUBSTANTIALLY ALL IN THE FORM OF ULTIMATE FIBERS AND OPENED-UP AGGREGATES OF ULTIMATE FIBERS, THE FELT HAVING BY WEIGHT NOT MORE THAN ABOUT 40% TOTAL MOISTURE CONTENT A PORTION OF WHICH IS DISTRIBUTED SUBSTANTIALLY UNIFORMLY THROUGHOUT THE INTERIOR FIBERS OF THE FELT AT A MOISTURE CONTENT IN THE RANGE FROM 8% TO 35% BY WEIGHT, THE MASS OF SAID FIBERS HAVING IN SUBSTANTIALLY UNIFORMLY DISTRIBUTION AT LEAST THE NATURAL WATERSOLUBLE CONTENT DERIVING FROM SAID WOOD; EXPOSING THE EDGES OF SAID FELT TO THE ATMOSPHERE AND DURING SAID EXPOSURE APPLYING MECHANICAL PRESSURE TO SAID FELT BETWEEN IMPERFORATE PRESSING SURFACES AND THEREBY EFFECTING A THICKNESS WHICH IS SUBSTANTIALLY THE THICKNESS AT WHICH THE FELT IS TO BE FINALLY CONSOLIDATED, SIMULTANEOUSLY TRANSMITTING HEAT THROUGH SAID SURFACES TOWARD THE FACES OF SAID FELT DURING SAID COMPRESSION FROM HEAT SOURCES HAVING TEMPERATURES IN THE RANGE FROM 300* TO 450* F., SAID THICKNESS BEING ATTAINED PRIOR TO THE TIME THE FELT BECOMES UNIFORMLY HEATED THROUGHOUT; HOLDING SAID MECHANICAL PRESSURE FOR A SHORT TIME WHILE GENERATED WATER VAPOR ESCAPES FROM THE FELT TOWARD AND THEN FACEWISE ALONG SAID SURFACES AND ALSO AT THE EXPOSED EDGES OF THE FELT, AND THE WATER-VAPOR PRESSURE WITHIN THE FELT INCREASES INWARDLY FROM THE EDGES IN THE PRESENCE OF A CORRESPONDINGLY INCREASING PROPORTION OF INTERSTITIAL FLUID CONSISTING SUBSTANTIALLY OF WATER AND DISSOLVED WATERSOLUBLES, DURING WHICH SAID SHORT TIME THE TEMPERATURE OF SAID FLUID INCREASES IN THE DIRECTIONS INWARDLY FROM THE EDGES OF THE FELT; LOWERING THE APPLIED MECHANICAL PRESSURE PRIOR TO THE ATTAINMENT OF THE BOILING POINT OF THE HOTTEST PORTION OF SAID FLUID, WHEREBY TO PERMIT INCREASED RATES OF VAPORIZATION OF WATER FROM THE FACES OF THE FELT AND EDGEWISE OF THE FELT WITH DEPOSITION OF SAID WATERSOLUBLES IN SITU AS A RESIDUE, THEREBY AVOIDING MOVEMENT AND MIGRATION OF SAID FLUID AS WOULD OCCUR WHEN PARTIALLY CONFINED BODIES THEREOF REACH THE BOILING POINT AND BOIL TO FORM STEAM IN PRESSURE POCKETS CAUSING SUCH MIGRATION; AND WHEN VOLATILIZATION OF WATER IS SUBSTANTIALLY COMPLETED WITH A RESULTING DEPOSITION OF SOLUTE IN SITU FOR USE AS BINDER SOLIDS, INCREASING THE MECHANICAL PRESSURE TO ATTAIN AND MAINTAIN FINAL THICKNESS TO BOND THE FIBERS IN PART AT LEAST BY SAID RESIDUE OF WATER-SOLUBLES AND TO REDUCE THE MOISTURE CONTENT TO A VALUE PERMITTING FULL RELEASE OF SAID PRESSURE WITHOUT RUPTURE OF THE RESULTING CONSOLIDATED BODY. 